A magneto-optical disk is developed as a rewritable optical disk, and some kind of it is already in practice as an external memory for a computer.
In a magneto-optical disk using a perpendicular magnetization film as a recording medium, recording and reproducing is practiced using light, so recording capacity can be larger than a floppy disk and a hard disk which use an in-plane magnetization film.
A recording density of a magneto-optical disk is limited by the size of a light beam spot on the magneto-optical disk. In short, when a diameter of a recording bit and a distance of the recording bits are smaller than the diameter of the light beam spot, plural recording bits get into the light beam spot, so the recording bits cannot be reproduced separately.
Therefore, in order to improve the recording density, the diameter of the light beam spot needs to be small, and it is effective to make a wave length of a laser beam used as a reproducing light beam short for this purpose. However, the shortest wave length of a semiconducter laser beam in present market is 680 nm, and a semiconducter laser with shorter wave length is being developed and not served yet. Accordingly, it is difficult to improve a recording density with a conventional magneto-optical disk.
Compared with this, for example, Jap. Jour. Appl. Phys., Vol. 31(1992) pp. 568-575 discloses a magneto-optical disk which is arranged laminated with a readout layer and a recording layer which show perpendicular magnetic anisotropy respectively. In the journal, two methods(RAP and FAD) are proposed which reproduce a recording bit from an area smaller than a laser beam spot, using the above-described magneto-optical disk.
The methods use a phenomenon that laser beam irradiation raises a temperature of an irradiated area and causes a temperature distribution that the closer it is to a center of the laser spot, the higher the temperature is. The above-described readout layer and the recording layer are so set as to have different magnetic properties, respectively, between the condition where it is in a range of higher temperatures and the condition where it is in a range of lower temperatures than such a predetermined temperature to distinguish a high-temperature area close to the center and a low-temperature area surrounding thereof(an area of room temperature).
FIG. 31 shows a structure explaining a principle of the above-described RAD(Rear Aperture Detection) type. As shown in this figure, this magneto-optical disk has a laminated magnetic double layer of a readout layer 91 and a recording layer 92. During reproducing, first, an initializing magnetic field Hinit is applied at room temperature. The value of the initializing magnetic field Hinit is set between a coercive force of the readout layer 91 at room temperature and a coercive force of the recording layer 92 which is larger than that of the readout layer 91. Accordingly, only the magnetization direction of the readout layer 91 is set to the direction of the initializing magnetic field Hinit, that is, initialized.
Next, a laser beam 93 is irradiated, while a reproducing magnetic field Hr is applied which is an external magnetic field with a direction reverse to the magnetization direction of the initializing magnetic field Hinit. At this time, an area close to the center of the area irradiated with the laser beam 93 has a high temperature above the above-described predetermined temperature. The reproducing magnetic field Hr is so set as to accord to a coercive force of the readout layer 91 and an exchange-coupling force applied by the recording layer 92 at the high temperature. Namely, the reproducing magnetic field Hr is so set that the sum of both the exchange-coupling force applied to the readout layer 91 and the reproducing magnetic field Hr is larger than the coercive force of the readout layer 91 at the high temperature. Therefore, the magnetization direction of the readout layer 91, which directed to the recording layer 92 after the initialization, turns over so as to direct to the direction to which the exchange-coupling force from the recording layer 92 acts when it is the high temperature as above-described. Thus, an information of the recording layer 92 is transcribed.
As a result, the recording layer 92 is masked by the readout layer 92 at a surrounding area having a lower temperature than the predetermined temperature in the area irradiated with the laser beam 93. Therefore, a recording bit can be reproduced only from the high-temperature area close to the center smaller than the diameter of the laser spot.
On the other hand, as shown in the FIG. 32, the above-described FAD(Front Aperture Detection) type magneto-optical disk is so arranged as to have an intermediate layer which shows a perpendicular anisotropy and a Curie temperature lower than the predetermined temperature 103, between a readout layer 101 and a recording layer 102.
In the above-described arrangement, an exchange-coupling force between the readout layer 101 and the recording layer 102 acts through the intermediate layer 103 at a low temperature. Therefore, a magnetization direction of the readout layer 101 is the same as that of the recording layer 102. During reproducing, similarly to the above-described, a laser beam 104 is irradiated and a reproducing magnetic field Hr is applied. At this time, the temperature of the intermediate layer 103 is higher than the Curie temperature in a high-temperature area SH, which is hotter than the predetermined temperature, close to a center in a laser spot S. Accordingly, an exchange-coupling force gets not to act between the readout layer 101 and the recording layer 102. So, the magnetization direction of the readout layer 101 is arranged to the direction of the reproducing magnetic field Hr, unrelated to the magnetization direction of the recording layer 102. Thus, the high-temperature area SH is masked by the readout layer 101.
Therefore, a recording bit is reproduced which is lacated at a surrounding area SL outside the high-temperature area SH close to the center of the area irradiated with the laser beam.(Note that the area SH is, for example, an area having a crescent moon shape which is outside a high-temperature area, because a shape of the high-temperature area SH is an ellipse as shown in the figure when the disk lotates.) Accordingly, an recording bit Rb can be reproduced from the surrounding area SL which is smaller than the laser spot diameter S. Thus, recording density in the beam running direction can be improved. A Japanese unexamined application 143041/1989 (tokkaiheil-143041) and a Japanese unexamined application 143042/1989 (tokkaiheil-143042) disclose a method which improves reproducing resolution(linear recording density) in the light beam running direction, by reproducing a recording bit while expanding and erasing the recording bit during reproducing. Moreover, a Japanese unexamined application 93058/1991 (tokkaiheil3-93058), a Japanese unexamined application 255941/1992 (tokkaihei4-255941), and a Japanese unexamined application 258372/1993 (tokkaihei5-258372) disclose a signal reproducing method which improves a track density as well as a density in the light beam running direction.
However, in the above-described RAD, two separately set external magnetic fields, that is, the initializing magnetic field and the reproducing magnetic field Hr are necessary according to each magnetic property of the readout layer 91 and the recording layer 92 both in the condition at room temperature and in the condition at a temperature higher than the predetermined temperature. So, the method has a problem that a recording-reproducing apparetus becomes large.
In the above-described FAD, the area SL having a crescent moon shapeoutside the area SH close to the center is concerned to reproducing in the laser spot. So, though recording density is improved in the disk running direction, crosstalk is easy to ocurr by invasion of a signal from a neighboring track when a track pitch is made narrow. Therefore, the method has a problem that it is difficult to improve a recording density in the direction along the disk diameter, that is, the direction perpendicular to the track.
On the other hand, in the methods disclosed in the Japanese unexamined application 143041/1989 (tokkaihei1-143041) and the Japanese unexamined application 143042/1989 (tokkaihei1-143042), linear recording density is improved, but, as crosstalk is the same as that in the conventional optical disk, it is difficult to improve a track density.
In the methods disclosed in the Japanese unexamined application 93058/1991 (tokkaihei3-93058) and the Japanese unexamined application 255941/1992 (tokkaihei4-255941). linear recording density and a track density are both improved, but a magnet to initialize the reproducing layer is necessary, so the apparatus grows large. In the method disclosed in the Japanese unexamined application 258372/1993 (tokkaihei5-258372), linear recording density and a track density are both improved, and a magnet to initialize the reproducing layer is not necessary. However, a large external magnetic field is necessary during reproducing, so the apparatus needs to be made large and more electric power is consumed.